The present invention relates in general to wireless telecommunication systems, and more particularly, to an apparatus, method and system for providing variable encoding levels, such as variable code rates, for encoding of in-band control messages in wireless telecommunication systems.
Various wireless telecommunication services utilize forms of in-band or out-of-band signaling for various message types. For example, in the North American TDMA (time division multiple access) Digital Radio Link protocol (American National Standards Institute (xe2x80x9cANSIxe2x80x9d) 136 protocol) for wireless telecommunication, each downlink carrier signal includes several potential time slots or frames for communication with multiple mobile units, such as mobile telephones. Each such downlink time slot or frame is further logically divided into traffic channels, such as for voice communication, and control channels, such as for paging and support services (such as short message services). Similar systems are also utilized internationally, such as within the GSM and IDEN protocols.
Within protocols such as ANSI-136, each downlink frame includes various fields which have been allocated to specific types of information, such as a fixed number of bytes (or other bandwidth allocation) for traffic data such as voice communication, and a fixed number of bytes for control information consisting of user information and Slow Associated Control Channel (SACCH) data. Such SACCH data may be distributed among many frames transmitted over time, e.g., over twenty time slots, providing for a comparatively slow transmission of such control information.
When faster transmission of control information is necessary or desirable, such as for transmission of handoff messages, a Fast Associated Control Channel (FACCH) is also utilized in the ANSI-136 protocol. For FACCH data, in band signaling is used, such that the FACCH data is transmitted within fields previously used and reserved for speech data, thereby preempting and effectively replacing the speech data which otherwise would have been transmitted. Generally for FACCH data, only two time slots are required for the transmission of the complete control message. In addition, because of potentially poor channel conditions with potentially high bit error rates, such as those which may occur in a handoff situation, FACCH data is also generally encoded utilizing a robust, high rate code, such as a xc2xc convolutional code, in an attempt to minimize bit error rates.
While such in-band signaling may be desirable for many circumstances, the corresponding loss of traffic data, such as the loss of voice communication data when replaced by FACCH data, may be highly undesirable under a wide variety of other circumstances, such as in an emergency situation of a xe2x80x9c911xe2x80x9d call. In addition, while utilization of FACCH data may be desirable for a variety of messages, such as transmission of important, emergency geolocation information, there also may be circumstances in which channel conditions are better and may not require such robust encoding, with its corresponding increased overhead and voice quality degradation.
As a consequence, a need remains to provide an apparatus, method and system for variable encoding levels, such as variable code rates, for encoding of in-band control messages, such as for FACCH data. Such variable encoding levels should provide for robust encoding of control messages under poor channel conditions, while providing another level of less robust encoding under other circumstances, such as in emergency situations or in more optimal channel conditions, to decrease control message overhead and minimize corresponding degradation or loss of traffic channel data, such as voice information.
The various method, apparatus and system embodiments of the present invention provide for variable levels (code rate) encoding of control information, such as FACCH data, which is transmitted in lieu of (in-band) traffic information, such as voice communication data. In addition to robust encoding of such data under poor channel conditions, a second level of less robust encoding is also provided for encoding of such data under other circumstances, such as in emergency situations, in more optimal or reasonable channel conditions, or in other circumstances in which avoiding a loss of voice information is desirable. The apparatus, method and system of the present invention, therefore, provide for the dynamic ability to decrease control message overhead and minimize corresponding degradation or loss of traffic channel data, such as voice information.
In the various embodiments, messages such as ANSI-136 handoff and geolocation messages may be encoded utilizing different code rates, and are then transmitted in lieu of voice data in a fast associated control channel (FACCH). The preferred system embodiment includes a transceiver (base station) and a mobile switching center. The transceiver provides for wireless transmission of an encoded message during the communication session. The mobile switching center includes instructions to create a message for transmission during a communication session. When the message is necessary for continuation of the communication session, such as a handoff message, or when channel conditions are unfavorable, or when the message has a comparatively shorter length, the mobile switching center includes instructions to select a first encoding level from a plurality of encoding levels, such as a robust or high code rate xc2xc convolutional code, and to encode the message using the first encoding level to form the encoded message for transmission. When the message is not necessary for continuation of the communication session, such as a geolocation message, or when channel conditions are favorable, the mobile switching center includes instructions to select a second encoding level from the plurality of encoding levels, such as a less robust or lower code rate xc2xd convolutional code, and to encode the message using the second encoding level to form the encoded message for transmission.
The mobile switching center includes further instructions to replace voice data with a portion of the encoded message for transmission of the portion of the encoded message within a field otherwise reserved for voice data, and to continue to replace voice data with successive portions of the encoded message for transmission of a corresponding portion of the encoded message within a field otherwise reserved for voice data until the entire encoded message has been transmitted.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.